Apparatus for fabricating self-assembling microstructures

ABSTRACT

Apparatus for assembling microstructures onto a substrate through fluid transport. The apparatus includes a vessel that contains the substrate, a fluid, and microstructures. The substrate has at least one recessed region thereon. The microstructures being shaped blocks self-align into the recessed regions located on the substrate such that the microstructure becomes integral with the substrate. The apparatus also includes a pump that circulates the microstructures within the vessel at a rate where at least one of the microstructures is disposed into the recessed region.

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under Grant (Contract)Nos. AFOSR-91-0327 and F49620-92-J-054-1 awarded by the Department ofDefense. The Government has certain rights to this invention.

CROSS-REFERENCE

This application is a continuation-in-part of application Ser. No.08/169,298, filed Dec. 17, 1993, now U.S. Pat. No. 5,545,291 in the nameof the present assignee. This application is also related to applicationSer. No. 08/485,478, filed on the same day as the present application,which is a continuation-in-part of application Ser. No. 08/169,298,filed Dec. 17, 1993, both in the name of the present assignee.

BACKGROUND OF THE INVENTION

The present invention relates to the field of electronic integratedcircuits. The invention is illustrated in an example with regard to themanufacture of gallium arsenide microstructures onto a siliconsubstrate, but it will be recognized that the invention will have awider range of applicability. Merely by way of example, the inventionmay be applied in the manufacture of devices containing silicon basedelectronic devices integrated with a gallium arsenide basedmicrostructures (or devices) such as light emitting diodes (LED),lasers, tunneling transistors, Gunn oscillators, integrated circuits,solar collectors, liquid crystal displays (LCDs), and others.

Industry currently needs a cost effective, efficient, and practicalmethod for assembling a higher cost microstructure onto a lower costcommercially available substrate. In particular, a material such asgallium arsenide possesses substantially better characteristics for somespecific electronic and opto-electronic applications rather thanmaterials such as silicon. However, in the fabrication of galliumarsenide devices, substantial regions of a gallium arsenide wafer aretypically unused and wasted. Such unused regions generally create aninefficient use of precious die area. In addition, processing galliumarsenide typically requires special techniques, chemicals, andequipment, and is therefore costly.

Other applications such as very large scale integrated (VLSI) circuitsmay be better fabricated in silicon rather than gallium arsenide. Instill further applications, it may be desirable to produce integratedcircuits having characteristics of both types of materials. Accordingly,industry needs to develop an effective method of fabricating a galliumarsenide device integrated with a silicon based integrated circuit. Theresulting structure of such method includes advantages of both galliumarsenide and silicon based devices.

Methods such as flip chip bonding, lift off methods, and others,generally require large areas of a substrate and are incompatible with amicron sized state-of-art microstructure. Such methods often createdifficulty in positioning a particle onto a substrate. Accordingly,industry needs to develop an effective method of fabricating higher costmaterials such as a gallium arsenide microstructure onto a lower costsubstrate such as silicon.

Industry utilizes or has proposed several methods for fabricatingindividual electronic components (or generally microstructures) andassembling such structures onto a substrate. One approach is to growgallium arsenide devices directly onto a silicon substrate. Thisapproach becomes limiting because the lattice structure of galliumarsenide mismatches that of silicon. In addition, growing galliumarsenide onto silicon is inherently difficult and therefore costly.Accordingly, gallium arsenide can not efficiently be grown on a siliconsubstrate.

Another approach is described by Yando in U.S. Pat. No. 3,439,416. Yandodescribes components or structures placed, trapped, or vibrated on anarray of magnets. Such magnets include magnetized layers alternatingwith non-magnetized layers to form a laminated structure. Components arematched onto the array of magnets forming an assembly thereof. However,severe limitations exist on the shape, size, and distribution of thecomponents. Component width must match the spacing of the magneticlayers and the distribution of components are constrained by theparallel geometry of lamination. In addition, self-alignment ofcomponents requires the presence of the laminated structure.Furthermore, the structures disclosed by Yando typically possessmillimeter sized dimensions and are therefore generally incompatiblewith micron sized integrated circuit structures. Accordingly, the methodand structure disclosed by Yando is thereby too large and complicated tobe effective for assembling a state-of-art microstructure or componentonto a substrate.

Another approach involves mating physical features between a packagedsurface mount device and substrate as described in U.S. Pat. No.5,034,802, Liebes, Jr. et al. The assembly process described requires ahuman or robotics arm to physically pick, align, and attach a centimetersized packaged surface mount device onto a substrate. Such process islimiting because of the need for the human or robotics arm. The human orrobotics arm assembles each packaged device onto the substrateone-by-one and not simultaneously, thereby limiting the efficiency andeffectiveness of the operation. Moreover, the method uses centimetersized devices (or packed surface mount integrated circuits), and wouldhave little applicability with micron sized integrated circuits in dieform.

Another approach, such as the one described in U.S. Pat. No. 4,542,397,Biegelsen et al. involves a method of placing parallelogram shapedstructures onto a substrate by mechanical vibration. Alternatively, themethod may also employ pulsating air through apertures in the supportsurface (or substrate). A limitation to the method includes an apparatuscapable of vibrating the structures, or an apparatus for pulsating airthrough the apertures. Moreover, the method described relies uponcentimeter-sized dies and would have little applicability withstate-of-art micron sized structures.

A further approach such as that described in U.S. Pat. No. 4,194,668 byAkyurek discloses an apparatus for aligning and soldering electrodepedestals onto solderable ohmic anode contacts. The anode contacts areportions of individual semiconductor chips located on a wafer.Assembling the structures requires techniques of sprinkling pedestalsonto a mask and then electromagnetic shaking such pedestals foralignment. The method becomes limiting because of the need for a shakingapparatus for the electromagnetic shaking step. In addition, the methodalso requires a feed surface gently sloping to the mask for transferringelectronic pedestals onto the mask. Moreover, the method is solely incontext to electrode pedestals and silicon wafers, thereby limiting theuse of such method to these structures.

Still another approach requires assembling integrated circuits onto asubstrate through electrostatic forces as described in application Ser.No. 07/902,986 filed Jun. 23, 1992 by Cohn. The electrostatic forcesvibrate particles such that the particles are arranged at a state ofminimum potential energy. A limitation with such method includesproviding an apparatus capable of vibrating particles with electrostaticforces. Moreover, the method of Cohn creates damage to a portion of theintegrated circuits by mechanically vibrating them against each otherand is also generally ineffective. Accordingly the method typicallybecomes incompatible with a state-of-art microstructure.

From the above it is seen that a method of assembling a microstructureonto a substrate that is compact, low cost, efficient, reliable, andrequires little maintenance is desired.

SUMMARY OF THE INVENTION

The present invention pertains to a method and resulting structure forassembling a microstructure onto a substrate. In particular, the methodincludes transferring shaped blocks or generally structures via a fluidonto a top surface of a substrate having recessed regions or generallybinding sites or receptors. Upon transferring, the blocks self-alignthrough their shape into the recessed regions, and integrate thereon.The resulting structure may include a variety of useful electronicintegrated circuits containing silicon based electronic devicesintegrated with a gallium arsenide based microstructures such as a lightemitting diodes (LED), lasers, tunneling transistors, Gunn oscillators,integrated circuits, solar collectors, and others. As an additionalexample, semiconductor devices may be integrated with othersemiconductor devices or other substrate materials, such as plastic.

In one specific embodiment, the method provides assembling amicrostructure such as a micron sized block onto a substrate. Thesubstrate includes a top surface with at least one recessed regionthereon and may be either a silicon wafer, gallium arsenide wafer, glasssubstrate, ceramic substrate, or others. The substrate may also be aplastic sheet fabricated from a technique such as stamping, injectionmolding, among others. Assembling steps include providing shaped blocks,and transferring the blocks into a fluid to form a mixture thereof orgenerally a slurry. Such slurry is then dispensed evenly over thesubstrate at a rate where at least one of the shaped blocks is disposedinto a recessed region. Dispensing occurs at substantially a laminarflow and allows a portion of the shaped blocks to self-align into therecessed region. Alternatively, the method includes circulating theslurry at a rate where at least one of the shaped blocks is disposedinto a recessed region. A gas, including but not limited to nitrogen,facilitates circulation of the slurry and allows a portion of the shapedblocks to self-align into the recessed regions.

The invention further provides related apparatus for assembling amicrostructure on a substrate with at least one recessed region thereon.The apparatus includes a vessel that contains the substrate, a fluid,and the shaped blocks. The apparatus also includes a pump thatcirculates the shaped blocks within the vessel at a rate where at leastone of the shaped blocks is disposed into a recessed region.

In an alternative embodiment, the method provides, for example, shapedblocks having a trapezoidal profile from an improved fabricationprocess. Fabrication includes providing a second substrate having a topsurface, and growing a sacrificial layer overlying the top surface. Astep of forming a block layer overlying the top surface is thenperformed. Masking and etching the block layer up to the sacrificiallayer creates trapezoidal shaped blocks thereon. A step of preferentialetching the sacrificial layer lifts off each trapezoidal shaped block.Such blocks are then rinsed and transferred into a solution forming theslurry. In this embodiment, the method provides alternate fabricationsof the shaped blocks. One such alternate fabrication of the shapedblocks includes providing a second substrate as a block layer having anupper surface and a bottom surface, and growing a sacrificial layeroverlying the upper surface. Another alternative fabrication of theshaped blocks includes providing a second substrate having an uppersurface, a sacrificial layer overlying the upper surface, and a blocklayer overlying the sacrificial layer. After providing the secondsubstrate, a step of forming trapezoidal shaped blocks on and in contactwith the sacrificial layer is then performed. The trapezoidal shapedblocks are then removed from contact with the sacrificial layer, andtransferred into a fluid to form a slurry.

The invention further provides a resulting trapezoidal shaped blockintegral with a substrate. The substrate includes a plurality ofrecessed regions thereon. Each recessed region includes a shaped profileto accept a trapezoidal shaped block. The resulting structure has suchblocks integrated with the substrate via the recessed regions formingassembled devices or integrated circuits.

Still a further embodiment, the shaped block comprises a truncatedpyramid shaped gallium arsenide structure. The truncated pyramid shapedstructure includes a base with four sides protruding therefrom to alarger top surface. Each side creates an angle between about 50° andabout 70° from the base to a side. Each side may also have a heightbetween about 5 μm and about 15 μm. The block may have a length betweenabout 10 μm and about 50 μm, and a width between 10 μm and about 50 μm.

The improved method and resulting structure are in context to atrapezoidal shaped block made of gallium arsenide assembled onto asilicon substrate merely for illustrative purposes only. The improvedmethod and related apparatus are in context to different sizedtrapezoidal shaped blocks made of silicon assembled onto a siliconsubstrate merely for illustrative purposes. The shaped blocks may alsoinclude a cylindrical shape, pyramid shape, rectangular shape, squareshape, T-shape, kidney shape, or the like (symmetrical andasymmetrical), and combinations thereof. Generally, the shape of theblock allows the block to closely insert into a similarly shapedrecessed region or receptor on a substrate. The shaped blocks alsocomprise a material such as gallium aluminum arsenide, silicon, diamond,germanium, other group III-V and II-VI compounds, multilayeredstructures, among others. Such multilayered structure may includemetals, insulators such as silicon dioxide, silicon nitride, and thelike, and combinations thereof.

A further understanding of the nature and advantages of the inventionwill become apparent by reference to the remaining portions of thespecification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a gallium arsenide wafer having a molecular beam epitaxy (MBE)grown gallium arsenide layer for the improved method of fabrication;

FIG. 2a is an illustration of trapezoidal shaped gallium arsenide blocksetched from an MBE grown gallium arsenide layer;

FIG. 2b is an illustration of reverse trapezoidal shaped blocks;

FIG. 3 is an illustration for a lift-off step of gallium arsenideblocks;

FIG. 4 is an illustration of a portion of an alternative lift-off stepusing a intermediate substrate;

FIG. 5 is an illustration of another portion of the alternative lift-offstep of FIG. 4;

FIG. 6 is an illustration of each gallium arsenide block self-aligningonto a silicon substrate;

FIG. 7 is an embodiment of a microstructure assembled onto the siliconsubstrate according to the improved method depicted by FIGS. 1-3 and 6;

FIG. 8 is an alternative embodiment of a microstructure assembled onto asubstrate;

FIG. 9 is an embodiment of a microstructure assembled onto a substrateforming a gallium arsenide diode;

FIG. 10 is an alternative embodiment of a microstructure assembled ontoa substrate forming a gallium arsenide diode;

FIG. 11 is a further alternative embodiment of a microstructureassembled onto a substrate forming a gallium arsenide diode;

FIG. 12 is an illustration of examples of shaped blocks;

FIG. 13 is a photograph of an assembled microstructure according to theexperiment; and

FIG. 14 is a photograph of an operational photo diode according to theexperiment.

FIG. 15 is a photograph of a metallized ring layer overlying a galliumarsenide block;

FIG. 16 is a current-voltage representation for a gallium arsenide diodeaccording to the experiment;

FIG. 17 is a current-voltage representation for a galliumarsenide/aluminum arsenide resonant-tunneling diode according to theexperiment;

FIG. 18 is a silicon wafer having a deposited silicon nitride layer foran alternate improved method of fabrication of shaped blocks;

FIG. 19 is a silicon-on-insulator wafer having a silicon-on-insulatorlayer overlying an insulator layer overlying a silicon substrate foranother alternate improved method of fabrication of shaped blocks;

FIG. 20 is an illustration of a mask used to fabricate silicon shapedblocks; and

FIG. 21 is an illustration of an apparatus for assemblingmicrostructures onto a substrate.

DESCRIPTION OF SPECIFIC EMBODIMENTS

With reference to FIGS. 1-21, the present invention provides an improvedmethod of fabricating a microstructure onto a substrate, a relatedapparatus, and an improved resulting structure. FIGS. 1-17 are, forexample, in context to fabricating and assembling a shaped galliumarsenide block onto a silicon substrate for illustrative purposes only.FIGS. 18-21 are, as further examples, in context to fabricating andassembling shaped silicon blocks onto a silicon substrate forillustrative purposes only.

In the assembly of a gallium arsenide block onto a silicon wafer,trapezoidal shaped blocks self-align into inverted trapezoidal shapedrecessed regions located on the top surface of the silicon wafer. Stepsfor such method include forming the gallium arsenide blocks,transferring the blocks into a solution forming a slurry, and spreadingthe slurry evenly over the top surface of a silicon substrate havingrecessed regions. During the spreading steps, the blocks self-align andsettle into the recessed regions while being transported with the fluidacross the top surface. Optionally, the slurry is spread evenly over thetop surface of a silicon substrate by way of mechanical means such as abrush, a scraper, tweezers, a pick, a doctor blade, and others. Themechanical means may be used to move or distribute the slurry and alsoto remove excess slurry. As an alternative to spreading the slurry, themethod includes circulating the slurry over the top surface of thesubstrate having recessed regions. During the circulating step, theblocks self-align and settle into the recessed regions while beingtransported with the fluid across the top surface. The blocks which donot settle into recessed regions are then recirculated until a certainfill-factor is achieved. Of course, the mechanical means may also beused in conjunction with the circulating step. The details offabricating the silicon substrate having recessed regions will bediscussed in detail below after a brief discussion of forming thegallium arsenide blocks. Details of the method using the circulatingstep and related apparatus will be discussed in detail below after adiscussion of fabricating the silicon substrate having recessed regions.

In a specific embodiment, the method provides as an example a step offorming trapezoidal shaped blocks from a gallium arsenide wafer. Suchstep includes providing a gallium arsenide wafer 10 as illustrated inFIG. 1. The method also provides forming sacrificial layer 13 bychemical vapor deposition, sputtering, or the like overlying top surface15 of gallium arsenide wafer 10. Such sacrificial layer 13 includes, forexample, aluminum arsenide. Other sacrificial layers may include indiumphosphate, silicon dioxide, photoresist, among other materials capableof being selectively etched. Of course, the sacrificial layer useddepends upon the particular application. For an aluminum arsenidesacrificial layer, thickness for such layer is between about 0.1 μm andabout 5.0 μm, and preferably at about 1 μm. Before forming sacrificiallayer 13, a step of etching top surface 15 by methods such as wetetching, plasma etching, or reactive ion etching clears off any nativeoxide. Alternatively, a step of desorption in the presence of arsenicremoves the native oxide layer. A subsequent step of preferentialetching (to be discussed in detail later) removes sacrificial layer 13to facilitate the lift-off of each gallium arsenide block (also called amesa shaped or trapezoidal shaped or truncated pyramid shaped structure)formed overlying sacrificial layer 13.

In FIG. 1, gallium arsenide layer 17 forms overlying sacrificial layer13. Such gallium arsenide layer may be fabricated by methods includingmolecular beam epitaxy, chemical vapor deposition, and others. Thethickness (T) of the gallium arsenide layer is at least about 10 nm andgreater, and preferably at about 10 μm and greater, depending upon theparticular application.

To produce the desired dimensions for the block, the improved methodprovides the steps of masking and etching gallium arsenide layer 17.FIG. 2a illustrates gallium arsenide substrate 10 after such masking andetching steps and includes gallium arsenide blocks 19 and a photoresistlayer 21 overlying gallium arsenide layer 17 (not shown). Generally,exposed portions of gallium arsenide layer 17 are etched up tosacrificial layer 13 as illustrated in FIG. 2a. Such etching stepprovides a plurality of shaped gallium arsenide blocks 19. For thepresent example, the shaped blocks include a trapezoidal profile ortruncated pyramid shape. Such trapezoidal profile may be fabricated bymethods of wet etching, plasma etching, ion milling, reactive ionetching, among others, depending on the application.

Generally, a wet etch produces a sloping profile along the sides oredges of each gallium arsenide block. For mask edges parallel to the110! direction, a wet etch produces an outward sloping profile asillustrated in FIG. 2a. Alternatively, mask edges parallel to the 110!direction, produces an inward sloping (or reverse mesa) profile. Theoutward sloping profile provides a desired shape which integrates into asilicon substrate having recessed regions shaped in a complementarymanner.

Ion milling produces gallium arsenide blocks with outward slopingprofiles, depending upon the beam angle. Beam angle is adjusted betweenabout 0° to about 30° from a normal to top surface 15 on galliumarsenide substrate 10. To create the outward sloping (or truncatedpyramid shape) profile for each block, the entire structure is typicallyrotated during such etching step.

Reactive ion etching (RIE) also produces blocks having a shaped profilein many materials such as gallium arsenide, silicon, or others. Ingeneral, a vertical etch may be used at a tilt to cut each sidewallangle. A substantially vertical etch process may also be tuned toundercut at a selected angle without tilting the substrate. In addition,a vertical etch or a reactive ion etch may be used in combination with awet chemical etch to provide the undercut. Such etching methods createblocks having undercut sides or a reverse mesa profile, as shown in FIG.2b. FIG. 2b illustrates reverse trapezoidal shaped blocks. FIG. 2b showssubstrate 925 (of material such as gallium arsenide, silicon, or thelike) after such masking and etching steps and includes shaped blocks929 on a sacrificial layer 927. A mask layer 933 overlies a device layer931 which includes electronic or other devices or portions of devices.Generally, unexposed portions of block layer 929 are etched throughsacrificial layer 927 as illustrated in FIG. 2b. Depending uponvariables such as the etchant, pressure, equipment, and others, suchetching method may create blocks having substantially consistent shapesand/or profiles. Vertical etches and wet etches may be used or combinedto produce such reverse mesa profiles. A plurality of shaped blocks 929are thereby produced by removal of the sacrificial layer 927. Removal ofthe sacrificial layer 927 may be accomplished for example by selectiveetching, chemical conversion (such as preferentially oxidizing thesacrificial layer, preferentially converting the sacricial layer intoporous Silicon, or others) followed by selective etching, thermal orultrasonic or mechanical fracture, or dissolving, among others, andpreferably by selective etching. Alternatively, the functionality of thesacrificial layer 927 may be included in the substrate 925, separatingthe blocks 929 by removal or etching of substrate 925 in whole or inpart.

In a specific embodiment, after etching the MBE grown layer, trapezoidalshaped blocks are removed through a lift-off technique from galliumarsenide substrate 10 by preferential etching sacrificial layer 13 asillustrated in FIG. 3. Such lift-off technique occurs by, for example, apreferential wet etch of the aluminum arsenide sacrificial layer. In thegallium arsenide example, such wet etching step is typically performedby a chemical such as a hydrofluoric acid solution or the like. Theetchant used substantially etches the sacrificial layer but does notaggressively attack gallium arsenide blocks and/or substrates.

After separating the gallium arsenide blocks from substrate 10, methodsof diluting and decanting the wet etchant solution remove the blocksfrom the solution. In the gallium arsenide example, the wet etchant isdiluted and decanted using purified water, methanol, ethanol, or thelike. Optionally, a rinsing step occurs after the diluting and decantingstep. The rinsing step relies on solutions such as acetone, methanol,ethanol, or any other inert solution having low corrosive properties.Such solution also provides a medium (or fluid) for creating a mixturehaving blocks suspended therein or generally a slurry.

Instead of the lift-off technique illustrated in FIG. 3, an alternativelift-off method creates intermediate structure 250 of FIG. 4 from thegallium arsenide structure of FIG. 2a. Such alternative lift-off methodalso promotes lift-off of the shaped blocks in applications where thedevices are formed onto the backside of the blocks. As shown, the methodincludes spreading a filler or wax layer 253 preferably high temperaturewax overlying the top surface of exposed portions of sacrificial layer13 and gaps 255 between each block 19. One such wax includes a productby the name of TECH WAX made by TRANSENE Co., Inc. The method thenincludes inverting the gallium arsenide structure of FIG. 2a andattaching top surface 21 onto intermediate substrate 257. Suchintermediate substrate is, for example, a silicon wafer or the like.However, prior to the attaching step, intermediate substrate surface 261undergoes steps of etching off any native oxide preferably with a wetetchant such as hydrofluoric acid, and treating the cleaned surface withan adhesion promotor such as hexamethyldisilazane also called HMDS. Inremoving gallium arsenide substrate 10, backside 263 is lapped untilabout 50 μm remains on substrate 10. The remaining thickness ofsubstrate 10 is then etched up to aluminum arsenide layer 13. An etchantsuch as ammonium hydroxide and hydrogen peroxide (6:200 NH₃ OH:H₂ O₂)preferentially etches the gallium arsenide substrate up to aluminumarsenide layer 13. Accordingly, the aluminum arsenide layer acts as anetch stop protecting gallium arsenide blocks 19. Removing aluminumarsenide layer 13 requires a step of wet etching using an etchant suchas hydrofluoric acid. Such etchant typically removes aluminum arsenidelayer 13 after a short dip in such solution. After the aluminum arsenidelayer is completely removed, steps including masking, sputtering, andetching form metallized ring contacts 265 as illustrated in FIG. 5. Suchmetallized ring contacts were made by patterns formed from photoresistlayer 267. The metallization for such contacts include materials such asgold, aluminum, among others. Alternatively, other processing steps suchas etching, masking, implantation, diffusion, and the like may beperformed on the blocks to create other profiles as well as activedevices thereon. A solution such as trichloroethane (TCA) dissolves thefiller or wax disposed between each block 19 and photoresist layer 21,and lifts off the gallium arsenide blocks 19 from intermediate substrate257. To decrease corrosion, the gallium arsenide blocks are transferredto an inert solution such as acetone, methanol, ethanol, or any othersolution having low corrosive characteristics. Such inert solution andblocks are often called a mixture or generally a slurry.

In another specific embodiment, the method provides as another examplesteps of forming trapezoidal shaped blocks from a silicon-type wafer asillustrated in FIG. 18. Such steps include providing a second substrate,such as a silicon wafer 700 defining a block layer 703 with a bottomsurface 705 and an upper surface 707. In this embodiment, silicon wafer700 is a single-side-polished wafer having a dimension of about 2" toabout 16" with a thickness of about 10 μm to about 2000 μm, and ispreferably a two inch silicon wafer of about 235 μm thickness. Bottomsurface 705 is the polished side of silicon wafer 700, and upper surface707 is the unpolished, rough side. The method also includes forming asacrificial layer 709 overlying upper surface 707 by a technique such aschemical vapor deposition, sputtering, molecular beam epitaxy, or thelike. Sacrificial layer 709 is a layer made of silicon nitride(SiN_(x)), silicon dioxide SiO₂, metals, or organics, with a thicknessof about 100 Å to about 100 μm, and is preferably a SiN_(x) layer ofabout 0.4 μm thickness. Similarly to the gallium arsenide blockfabrication example, masking and etching steps can be used to form outof block layer 703 the trapezoidal shaped blocks or truncated pyramidshapes. In this embodiment, the trapezoidal shaped block or truncatedpyramid shaped structure includes a base with four sides protrudingtherefrom to a larger top surface. Each side creates an angle betweenabout 20° and about 90° from the top surface to a side, and preferablyis about 55°. The block may have a length between about 1 μm and about 1cm, and a width between about 1 μm and about 1 cm, and preferably has alength of about 1.0 mm and a width of about 1.2 mm. The larger face ison and in contact with sacrificial layer 709. A subsequent step ofpreferential etching removes sacrificial layer 709 to free eachtrapezoidal shaped block formed overlying sacrificial layer 709. Theblocks are then transferred into a fluid to form a slurry containing thelarger trapezoidal shaped blocks.

In yet another specific embodiment, the method provides as anotherexample steps of forming trapezoidal shaped blocks from asilicon-on-insulator (SOI) wafer as illustrated in FIG. 19. Such stepsinclude providing a second substrate such as a SOI wafer 800, thatdefines a silicon substrate 803 with an upper surface 805, a sacrificiallayer 807 overlying upper surface 805, and a block layer 809 overlyingsacrificial layer 807. Sacrificial layer 807 composed of SiO₂ is aninsulator layer of SOI wafer 800 and block layer 809 composed of siliconis a silicon-on-insulator layer of SOI wafer 800. In this embodiment,SOI wafer 800 has a silicon-on-insulator layer of a thickness of about 1μm to about 2000 μm, and an insulator layer of thickness of about 100 Åto about 1000 μm, and preferably has a silicon-on-insulator layer ofabout 35 μm thick silicon, and an insulator layer of about 0.4 μm thickSiO₂. Similarly to the other block fabrication examples, masking andetching steps can be used to form out of block layer 809 the trapezoidalshaped blocks on and in contact with sacrificial layer 807.

In this embodiment, the trapezoidal shape block or truncated pyramidshaped structure includes a base with four sides protruding therefrom toa larger top surface. Each side creates an angle between about 20° andabout 90° from the top surface to a side, and preferably is about 55°.The block may have a length between about 1 μm and about 2000 μm, and awidth between about 1 μm and about 2000 μm, and preferably has a lengthof about 150 μm and a width of about 150 μm. As in the silicon examplehaving larger trapezoidal shaped blocks, the larger face of the smallertrapezoidal shaped blocks is in contact with sacrificial layer 807. Asubsequent step of preferential etching removes sacrificial layer 807 tofree each trapezoidal shaped block formed overlying sacrificial layer807. Of course, trapezoidal shaped blocks can be removed from contactwith the sacrificial layer by steps such as preferential etching, ionmilling, or dissolving the sacrificial layer. The blocks are thentransferred into a fluid to form a slurry.

In the two preceding examples of forming trapezoidal shaped blocks fromsilicon, the slowest etching planes for silicon in the KOH:H₂ O etchingsolution used are the {111} planes, which can be considered etch stopsforming the sloping sides of the shaped blocks. In each silicon example,the respective mask used to define the blocks must be aligned to theappropriate crystal axis. As shown in FIG. 20, a mask 850 was used toform the silicon trapezoidal blocks 855. Silicon trapezoidal shapedblock 855 is formed at the intersection of the diagonal lines on mask850, and the larger face 860 of silicon trapezoidal shaped block 855 isin contact with sacrificial layer (respectively 709 and 807, for thefirst and second preceding silicon examples). The width of the diagonalmask lines in FIG. 20 must be twice the thickness (t) of silicon blocklayer. For the larger silicon trapezoidal blocks, a=0.2 mm and siliconblock layer 703 has t=235 μm; whereas, for the smaller silicontrapezoidal blocks, a=0 and silicon block layer 809 has t=35 μm.

For each preceding silicon example, etching is completed when siliconblock layer 703 or 809 is etched entirely through, and simultaneouslywhen the corners are precisely formed. Continuing etching beyond thispoint does not change the overall dimensions of the trapezoidal shapedblock, but merely rounds the corners. Because of geometricconsiderations, the width of the top face of the shaped block must be atleast 3√2 times the thickness of the silicon block layer. This limitsthe aspect ratio of the blocks fabricated by this technique. This maskpattern utilizes as high as 50% of the silicon area if there is nodistance between block corners.

In the first silicon example with the larger shaped blocks, the etchedsilicon wafer is placed in concentrated HF etch solution to remove theshaped blocks from contact with the SiN_(x) sacrificial layer and anyremaining SiN_(x) from the mask layer. In the second silicon examplewith the smaller shaped blocks, the etched SOI wafer similarly is placedin concentrated HF etch solution to remove the shaped blocks fromcontact with the SiO₂ sacrificial layer and any remaining SiN_(x) fromthe mask layer. This HF etch solution preferentially etches the SiO₂ andthe SiNx to free the shaped blocks without etching the silicon shapedblocks. In particular, a HF solution having a concentration of about 1:1HF:H₂ O was used to etch the sacrificial layer and residual SiNx to freethe shaped blocks.

The slurry comprises an inert solution (of fluid) and shaped blocks.Enough solution exists in the slurry to allow the blocks to slide acrossthe top surface of the substrate. Preferably, the amount of solution inthe mixture is at least the same order as the amount of blocks. Ofcourse, the amount of solution necessary depends upon characteristicssuch as block size, block material, substrate size, substrate material,and solution. After preparation, the slurry is transferred or spreadover top surface 53 of silicon substrate 50 as illustrated in FIG. 6.The details of the transferring technique are discussed below after abrief discussion in fabricating silicon substrate 50.

As shown in FIG. 6, silicon substrate 50 comprises etched recessedregions 55. A variety of techniques including wet etching, plasmaetching, reactive ion etching, ion milling, among others providerecessed regions 50, or generally trenches, receptors, or binding sites.Such techniques etch recessed regions 50 with a geometric profile whichis complementary to block 19. In the silicon substrate, for example,each recessed region includes a trapezoidal profile or invertedtruncated pyramid shape. The trapezoidal profile allows block 19 toself-align and fit closely into recessed region 50 via the improvedtransferring technique.

The transferring technique includes a step of evenly spreading orpouring the slurry over top surface 53. The transferring technique maybe accomplished by pouring a vessel of slurry evenly over top surface53. Alternatively, the slurry may also be transferred from a pipet,flask, beaker, or any other type of vessel and/or apparatus capable ofevenly transferring the slurry over top surface 53. Generally, theslurry is poured over top surface 50 at a rate which allows substantialcoverage of the top surface, but prevents blocks already disposed intothe recessed regions from floating or popping out. Slurry flow istypically laminar but can be non-laminar, depending upon the particularapplication. In the gallium arsenide block example, the fluid flux overtop surface 53 occurs at a velocity between about 0.01 mm/sec. and about100 mm/sec. Preferably, fluid flux occurs at about 1 mm/sec. At suchflux rates, the blocks flow evenly with the fluid, tumble onto topsurface 53, self-align, and settle into recessed regions 55. Optionally,to prevent the blocks already disposed in the recessed regions fromfloating out, the transferring step may take place in a centrifuge orthe like. A centrifuge, for example, places a force on the blocksalready disposed in the recessed regions and thereby prevents suchblocks from floating out with solution.

Alternatively, the transferring technique may be accomplished by amethod and related apparatus that includes circulating the slurry overtop surface at a rate which allows blocks to dispose into the recessedregions. The rate may be adjusted to permit the blocks to fill therecessed regions so a desired fill-factor is achieved. The slurry isrecirculated at a rate so that blocks which are not already disposedcontinuously flow across top surface until a certain fill-factor isattained. Optimally, the slurry is recirculated at a rate which does notdisturb blocks already disposed into recessed regions.

The circulation rate is adjustable to accomodate a desired fill-factor,which will vary depending on the size of the blocks, number of recessedregions, and the specific application. Some applications require theintegration of silicon circuits onto transparent substrates such asglass or plastic for use in an active matrix flat panel display. Eachpixel or pixel element would have a small corresponding circuitassembled by the method. Larger circuits could also be assembled, suchas for multichip modules which might require a number of differentlarger circuits on the same substrate. Each of these circuits could beetched to a specific shape and assembled by the method into matchingrecessed regions. Different applications can also require differentfill-factors. For example, multichip modules could tolerate lowerfill-factors than flat panel displays, because of the difference innumber of components. The method and apparatus of the present inventionexhibit high fill-factors for different sizes of shaped blocks. Afterthe shaped blocks have been assembled onto the substrate, the disposedblocks can then be bonded and planarized, or the like, if necessary.

In order to continuously flow shaped blocks across the substrate, anapparatus circulates the slurry containing the shaped blocks across thesubstrate. The shaped blocks and fluid circulate within the apparatusand generally tumble onto the top surface of the substrate which is alsocontained in the apparatus. The shaped blocks self-align and engage witha recessed region having a is complementary shape. The shaped blockswhich are not disposed into recessed regions flow off the substrate andenter the recirculation path of the apparatus to flow over the substratefor disposition into recessed regions until a certain fill-factor isachieved.

As illustrated in FIG. 21, an apparatus 900 includes a vessel 903 and apump 905. In this particular embodiment, apparatus 900 was made entirelyof glass but may be constructed of other suitable materials. Vessel 903includes a receptacle 907 and a conduit 909. Vessel 903 contains asubstrate 911 having recessed regions and the mixture of fluid andshaped blocks. Conduit 909, coupled to receptacle 907 which containssubstrate 911, includes an input 913, an output 915 leading back toreceptacle 907, and a column 917 coupled at one end to input 913 and atthe other end to output 915. Pump 905 is coupled to input 913 anddispenses a gas into conduit 909 to facilitate circulation of the fluidand shaped blocks over substrate 911 at a rate where at least one shapedblock is disposed into a recessed region.

Receptacle 907 includes a holder 919 and a funneled bottom 921. Holder919 secures substrate 911 and is capable of moving substrate 911 tofacilitate the filling of the recessed regions. Additionally, holder 919agitates or orients substrate 911 so that shaped blocks not disposed inrecessed regions can flow off substrate 911 and back into arecirculation path in vessel 903. The bottom of receptacle 907 isfunneled to cause the shaped blocks not disposed into recessed regionson substrate 911 to fall to the bottom of receptacle 907 forrecirculation through vessel 903. Shaped blocks not disposed intorecessed regions then recirculate through vessel 903.

More specifically, funneled bottom 921 is coupled to input 913 ofconduit 909. Shaped blocks not disposed into recessed regions tumble tofunneled bottom 921 to input 913 of conduit 909, where pump 905dispenses a gas, such as nitrogen. The injected gas forms bubbles withinthe fluid inside column 917 of conduit 909. A gas bubble transports aportion of the fluid and at least one shaped block funneled to input 913through column 917 back to receptacle 907 through output 915 forattempted disposition into a recessed region. The gas bubble risesthrough column 917 and transports the fluid and the shaped block tooutput 915 leading back to receptacle 907.

In this embodiment, apparatus 900 uses nitrogen bubbles to circulate thefluid and the shaped blocks over substrate 911 without damaging theshaped blocks. Depending on, among other factors, the material of theshaped blocks and the fluid used, apparatus 900 may use other media, orgases such as air, hydrogen (H₂), nitrogen (N₂), oxygen (O₂), or argon(Ar) which do not damage the blocks or otherwise react with the fluid.Pump 905 dispenses the gas or transport medium which can be changed toachieve different fill-factors of the recessed regions.

In a specific embodiment, the resulting structure 20 of the methoddescribed by FIGS. 1-3 and 6 is illustrated in FIG. 7. The assembledmicrostructure includes silicon substrate 10, gallium arsenide blocks19, and recessed regions 55. The trapezoidal shape of the blocks andrecessed regions allows a block to self-align and fit closely into arecessed region during the transferring step. An angle (A) formedbetween one side of the block and the corresponding side of the recessedregion is between about substantially 0° to about 20°. Preferably, suchangle is less than about 5° but greater than substantially 0°. Suchangle facilitates the self-alignment process of each block. The improvedmethod allows for the fabrication of multiple blocks or microstructuresonto a substrate by various shaped blocks and recessed region geometriesand the fluid transferring step.

In a modification to the preceding specific embodiment, the blocks 19are attached into recessed regions 55 through eutectic layer 75 asrepresented in structure 70 of FIG. 8. Prior to the lift-off step, ametallized layer such as gold, silver, solder, or the like is formedonto surface 73. Alternatively, the layer attaching the block with eachrecessed region may be a synthetic adhesive or the like instead of aeutectic layer. Process steps comprising masking, etching, andsputtering typically form such metallized layer. Subsequent to thetransferring step, heating structure 70 forms eutectic layer 75 betweenmetallization layer 73 and silicon substrate 10. The eutectic layerprovides both mechanical and electrical contact between substrate 10 andblock 19. The method of attaching the blocks onto the substrate providesan efficient, cost effective, and easy technique.

In an alternative specific embodiment, the portions of the improvedmethod of FIGS. 1, 2, 4, 5, and 6 provides the resulting galliumarsenide light emitting diodes (LED) 200 as illustrated in FIG. 9. Asshown, the gallium arsenide LED includes silicon substrate 203 andgallium arsenide block 205. Each gallium arsenide block includes atleast metallized ring contacts 207, p-type gallium arsenide layer 209,n-type gallium arsenide layer 211, and eutectic layer 213. To illuminatethe device, voltage is applied to metallized ring contact 207 ormetallization layer. Photons (hν) are illuminated from a center regionwithin each metallized ring contact 207 of gallium arsenide block 205 asshown.

In a further alternative specific embodiment, the improved structureforms gallium arsenide light emitting diodes (LED) 90 as depicted inFIG. 10. Like the previous embodiment, the gallium arsenide LED includessilicon substrate 93 and gallium arsenide block 95. Each galliumarsenide block also includes at least metallized surface 97, p-typegallium arsenide layer 101, n-type gallium arsenide layer 103, andeutectic layer 105, similar to the preceding embodiment. To illuminatethe device, voltage is applied to metallization layer 97 by, forexample, a probe. Photons (hν) are illuminated from an edge regioninstead of a center region of gallium arsenide block 95 as shown.

Still in another specific embodiment, the improved structure formsgallium arsenide structure 120 having tapered aperture opening 123 asillustrated in FIG. 11 (not to scale). A process step such as wetetching, ion milling, reactive ion etching, and others forms the taperedaperture opening 123. The gallium arsenide structure may be an LED,laser, or the like. Similar to the previous embodiment, gallium arsenidestructure 120 includes substrate 125 and gallium arsenide block 127.Structure 120 also includes a top metallization layer 131 such asaluminum overlying gallium arsenide block 127 and an insulating layer133. A ring contact layer 135 provides mechanical and electrical contactbetween substrate 125 and gallium arsenide block 127. Mechanical supportand electrical contact for the gallium arsenide block comes from ledge137. Also shown is a light emitting (or lasing) aperture 139 having adimension between about 5 μm and about 40 μm. To turn-on the device,voltage is applied to metallization layer 131. Photons (hν) illuminatefrom gallium arsenide block 127, through light emitting aperture 139,and through tapered aperture opening 123 as shown. Fiber optic cable 141receives the photons.

The improved method and resulting structure are in context to atrapezoidal shaped block made of gallium arsenide or silicon merely forillustrative purposes only. Alternatively, the improved method andstructure can be in context to almost any block having shaped features.Shaped features allow such blocks to move over the surface of thesubstrate via fluid transport, align with a corresponding recessedregion, and insert into such recessed region. FIG. 12 illustratesfurther examples of the shaped blocks. As shown, the blocks may, forexample, include a rectangular shape 300, octagonal shape 303, orcircular shape 305. The rectangular shaped block includes up to fourorientations for insertion into a substrate having a correspondingrecessed region. Alternatively, the octagonal shaped block includes upto eight orientations and the circular shaped block includes continuousorientations as long as the narrow end inserts first into the recessedregion. Such blocks may also comprise a material such as silicon,gallium arsenide, aluminum gallium arsenide, diamond, germanium, othergroup III-V and II-VI compounds, multilayered structures, among others.Such multilayered structures may include metals, insulators such assilicon dioxide, silicon nitride and the like, and combinations thereof.Generally, the block can be made of almost any type of material capableof forming shaped features. Typically, such blocks are fabricated bymethods including ion milling, reactive ion etching, and the like. Infacilitating alignment of each block onto a recessed region, an anglebetween a side of the block and the corresponding side of the recessedregion for a disposed block is between about substantially 0° to about20°. Preferably, such angle is less than about 5° but greater thansubstantially 0°.

The shaped block assembles with a substrate such as a silicon wafer,plastic sheet, gallium arsenide wafer, glass substrate, ceramicsubstrate, or the like. The substrate includes almost any type ofmaterial capable of forming shaped recessed regions or generally bindingsites or receptors thereon which complement the shaped blocks.

EXAMPLES

To prove the principle and demonstrate the operation of the method andstructure, a gallium arsenide block in the form of a diode was assembledonto a silicon substrate and operated.

In a gallium arsenide example, a slurry including gallium arsenideblocks were transferred such that the blocks self-aligned into recessedregions located on a top surface of a silicon substrate. The steps forsuch method included forming the gallium arsenide blocks, transferringthe blocks into a solution forming a slurry, and transporting the slurryevenly over a top surface of a silicon substrate having recessedregions. The shaped blocks generally tumble onto the top surface of thesubstrate, self-align and engage with a recessed region having acomplementary shape.

In creating the silicon substrate, a solution of ethylenediaminepyrocatechol pyrazine (EDP) or potassium hydroxide (KOH) producedrecessed regions having a trapezoidal profile or inverted truncatedpyramid shape. Each solution created trapezoidal shaped profiles havingan outward slope of about 55° from an angle normal to the top surface ofthe substrate. Trapezoidal profiles occurred due to the selectivity(1:100) between the {111} plane and the {100} or {110} plane.Specifically, the {111} plane etched slower than the {100} or {110}plane by a ratio of 1:100.

In the present example, an EDP solution etched recessed regions into asilicon substrate. EDP includes ethylenediamine (about 500 ml.),pyrocatechol (about 160 gms.), water (about 160 gms.), pyrazine (about 1gm.). The EDP bath was also at a temperature of about 115° C. Prior tothe etching step, a thermal oxide (SiO₂) layer having a thickness ofabout 200 nm was first formed on a top surface of such substrate.Masking and etching such oxide layer formed rectangular shaped regions.Such regions were then etched vertically about 10 μm forming squareopenings on the top surface about 23 μm in length. Sides protrude downsymmetrically from each opening to a square base having a length ofabout 9 μm.

In fabricating trapezoidal shaped blocks, an epi-ready two inch n-typegallium arsenide wafer provided a substrate for the formation of theself-aligning blocks. Native oxide on the top surface of such block wasfirst cleared off by a desorption process. The desorption processincluded exposing the wafer to a temperature of about 700° C. andelements including arsenic. After the desorption step, a sacrificiallayer comprising 1 μm of doped or undoped aluminum arsenide was grown onand in contact with the top surface. A thickness of about 10.7 μm ofsilicon doped gallium arsenide was then grown through an MBE processoverlying the aluminum arsenide layer. Silicon dopants were at aconcentration of about 10¹⁸ atoms/cm³. The top surface of the MBE grownlayer was then patterned with photoresist.

Patterning the top surface of the MBE grown layer included spreading aphotoresist layer having a thickness of about 1.6 μm over the topsurface of the MBE grown gallium arsenide layer. The photoresist used isa product made by Shipley under the name of AZ1400-31. Patterning stepsalso included at least exposing, developing, and baking the photoresist.Such baking step occurred at a temperature of about 120° C. for about 1hour to hard-bake the photoresist layer. The patterning steps formed aplurality of rectangles each having a dimension of about 35 μm by 24 μm(exposed portions of the photoresist) on the top surface.

After patterning, unexposed regions were etched forming trapezoidalshaped blocks attached to the aluminum arsenide sacrificial layer.Proper fit between the block and the recessed region requires each blockto have substantially the same shape. Accordingly, variousconcentrations and techniques of wet etching were tested in thisparticular example.

Generally, wet etching the unexposed regions produced results whichdepended upon the orientation of the mask edges. If the mask edges wereparallel to the 110! direction, wet etching the unexposed regionscreated outward sloping profiles from the top surface of each block.Alternatively, wet etching unexposed regions where mask edges wereparallel to the 110! direction created inward sloping (or reverse mesa)profiles.

Wet etching produced such different profiles (mesa and reverse mesa)because gallium arsenide includes two distinct sets of {111} planes. Ina {111} A or {111} gallium plane, each gallium atom on the surface hasthree arsenide atoms bonded below. For a {111} B or {111} arsenic plane,each arsenide atom on the surface includes three gallium atoms bondedbelow. Each arsenide atom in the {111} B layer includes a pair ofdangling electrons and is therefore exposed. Such dangling electrons arenot present in the structure of the {111} A plane. Accordingly, {111} Bplanes tend to etch faster than {111} A planes, thereby forming blockshaving a reverse mesa shape which is generally incompatible with therecessed regions etched on the silicon substrate.

Mask edges parallel to the 110! plane produced more undercutting thanthe cases where mask edges were parallel to the 110! plane. In thepresent example, mask edges parallel to the 110! direction producedabout 1.1 μm of horizontal etching per micron of vertical etching nearthe tops of the blocks. Regions near the base of the blocks producedetches of about 0.4 μm of horizontal etching per micron of verticaletching. Alternatively, mask edges parallel to the 110! plane producedetches of about 0.8 μm of horizontal etching per micron of verticaletching for regions near the top of the blocks, and 0.1 μm of horizontaletching per micron of vertical etch near the bottom of the blocks. Theformation of a square region at the base required a longer mask in the110! direction.

In addition to mask alignment, etchant concentration also affected theshape of each gallium arsenide block. A solution of phosphoric acid,hydrogen peroxide, and water (H₂ PO₃ :H₂ O₂ :H₂ O) provided a promisingetchant for the MBE grown gallium arsenide layer in the present example.Such etchant created three distinct profiles, depending upon the amountof hydrogen peroxide and water added to phosphoric acid. Diluteconcentrations of phosphoric acid (1:1:40 H₂ PO₃ :H₂ O₂ :H₂ O) created atrapezoidal or mesa shaped profile having a 30° angle between the topsurface of the block and a corresponding side. Etchant solutions whichwere less concentrated produced shallower trapezoidal or mesa shapedprofiles at angles from about 10° to 20°. Such shallower profiles wereprobably a result of etching reactions being transport limited in the{111} B planes.

Higher concentrations of phosphoric acid (1:1:20 H₂ PO₃ :H₂ O₂ :H₂ O andabove) created inward sloping (or reverse mesa) profiles limited by thereaction of the {111} B planes. Preferably, a phosphoric acidconcentration (1:1:30 H₂ PO₄ :H₂ O₂ :H₂ O) between the dilute andconcentrated solutions provides better profiles for assembly withrecessed regions etched on the silicon substrate. Such etchant producedblocks having angles of 55° parallel to the 110! plane and 49° parallelto the 110! plane, and typically etched the MBE grown layer at a rate ofabout 0.133 μm/minute (or about 133 nm/min). In producing the resultsdescribed, etchant solution was typically replenished when depleted.

Increasing the ratio of phosphoric acid to hydrogen peroxide by 3:1produced similar profiles to the experiments described, but generallycaused rough surfaces on the sides. Such rough surfaces were desirablefor the present application.

In a modification to this example, a similar wet etchant (1:1:30 H₂ PO₃:H₂ O₂ :H₂ O) facilitated the formation of aluminum gallium arsenideblocks from an aluminum gallium arsenide MBE grown layer. Such etchantprovided an inward sloping profile parallel to the 110! direction for analuminum gallium arsenide (x=0.1, Al_(x) Ga_(1-x) As) grown MBE layer.Vertical etch rates were about the same as the gallium arsenide MBEgrown layer. However, the presence of aluminum arsenide increasedetching of the {111} B plane into the reaction-rate limited regime. Suchetchant produced an inward sloping profile because etching x=0.1, Al_(x)Ga_(1-x) As was more reactive in the {111} B plane than galliumarsenide.

In addition to wet etching, ion milling was also used to create thegallium arsenide trapezoidal shaped blocks. Ion milling the MBE growngallium arsenide layer provided outward sloping profiles ranging atangles of about 68° to 90° between the top surface and a correspondingside. To produce such angles, the ion beam angles ranged from about 0°to 25° in reference to a normal from the top surface of the MBE grownlayer. Steeper beam angles (closer to 90°) generally created vertical orsubstantially vertical profiles. Ion milling also required the substrateto be rotated about a center axis during such processing step. Otherprocessing variables included an argon gas etchant, pressure of about 50millitorr, ion energy of about 1000 v, and an ion milling rate of 1 μmevery seven minutes. As the photoresist mask eroded laterally about 5 μmevery 70 minutes during milling, sidewalls having angles at about 68°were produced. Selectivity between the gallium arsenide and photoresistwas about 3:1. Ion milling produced substantially consistent galliumarsenide blocks and was therefore more effective than wet etching inthis particular example.

A final bath having a concentration of 1:1:30 H₂ PO₃ :H₂ O₂ :H₂ O wasused to clear off remaining oxides of either gallium arsenide andaluminum arsenide. Such oxides were typically formed when aluminumarsenide was exposed to etching baths or ion milling. Hydrofluoric acidmay then be used to clear off the oxide layers (typically rough lookingand brown in appearance). Generally, such oxide layers reduce theeffectiveness of hydrofluoric acid (HF) etching on the sacrificialaluminum arsenide layer.

After clearing off any oxide layers, a HF solution preferentially etchedthe sacrificial layer of aluminum arsenide to lift-off the galliumarsenide blocks. In particular, a HF solution having a concentration ofabout 5:1 H₂ O:HF was used to etch the sacrificial layer and lift offthe blocks. Any blocks still remaining on the substrate possibly throughsurface tension can be mechanically removed from the substrate into asolution. Removed blocks include a base dimension of about 22 μm by 23μm, compared to a designed dimension of 24 μm by 24 μm.

After removing the blocks from the substrate, a Teflon pipet was used toremove a substantial portion of the HF solution from the galliumarsenide blocks. Any remaining HF was rinsed off with water. Suchrinsing step created a mixture including blocks and water. An inertsolution such as acetone then replaced the water to decrease any oxideformation on the blocks. Once in the inert solution, the blocks maycluster together and either float to the surface or settle to the bottomof the solution. Such clusters, often visible to the naked eye,decreased the effectiveness of a subsequent transferring step, and weretherefore separated by mechanically agitating the solution withultrasonic vibration.

The inert solution including gallium arsenide blocks was thentransferred (or poured) evenly over the top surface of the siliconsubstrate. In particular, a pipet was used to transfer such solutionover the top surface of the substrate. The solution is transferred at arate creating substantially a laminar flow. Such laminar flow allowedthe blocks to tumble and/or slide onto the top surface of the substrateand then self-align into the recessed regions via the trapezoidalprofile. Generally, the transfer rate should provide an even flow ofsolution including blocks over the substrate surface but should not freeor remove any blocks already disposed into the recessed regions.

Blocks fabricated by ion milling produced higher yields than wet etchedblocks. Ion milled blocks having substantially consistent profiles selfaligned and inserted into more than 90% of the recessed regions locatedon the substrate surface before the solution substantially evaporated.As the solution evaporates, surface tension often pulled a portion ofthe blocks out of the recessed regions. About 30% to 70% of the recessedregions remained filled after evaporation. The decrease in yield can beaddressed by using liquids having lower surface tension duringevaporation or by super critical drying methods which substantiallyeliminates surface tension. Alternatively, blocks may be bonded into therecessed regions prior to evaporation of the solution, thereby fixingthe yield. Wet etched blocks having less consistent block profilesinserted correctly into about 1% to 5% of available recessed regions.Accordingly, ion milled blocks provided higher yields relative to theblocks fabricated by wet etching.

Photographs shown in FIG. 13 illustrate gallium arsenide blocks disposedinto recessed regions of the silicon substrate 150 according to thepresent example. A top portion 153 of each recessed region is square andmeasures about 23 μm at length. As shown, the photograph includesrecessed regions 155, silicon substrate 157, and trapezoidal shapedblock 159.

To further illustrate the operation of the present example, anilluminated diode 170 is shown in the photograph of FIG. 14. Thephotograph includes silicon substrate 173 and illuminated galliumarsenide LED 175. The gallium arsenide LED emitted infrared radiationwhile under electrical bias. Each gallium arsenide LED which was grownon an MBE layer included an N+ gallium arsenide cap layer (about 100 nmthickness), an N+ Al₀.1 Ga₀.9 As transport layer (about 1 μm thickness),a P- active region (about 1 μm thickness), and a P+ buffer layer (about1 μm thickness). The gallium arsenide LED also required a ringmetallized contact 400 for applying voltage and an opening 403 for lightoutput at a top portion of each block as illustrated in FIG. 15. Acurrent-voltage (I-V) curve 500 illustrated in FIG. 16 exhibits typicalp-n junction characteristics for the gallium arsenide structure of FIG.14.

Gallium arsenide/aluminum arsenide resonant-tunneling diodes (RTD's)were also integrated onto silicon. RTD's grown on an MBE layer includegallium arsenide wells (depth at about 5.0 nm) between two aluminumarsenide barriers (depth at about 2.5 nm). Current-voltagecharacteristics 600 for the RTD's integrated with silicon exhibitedproper differential negative resistance (NDR) at V_(peak) =2.0 v. asillustrated in FIG. 17. At such voltage, peak-to-valley ratio was about2.5. Oscillations (rf) observed after biasing the RTD's in the NDRregion were limited to about 100 MHz. External capacitances andinductances of the biasing circuit caused such limitations in frequency.

The description above is in terms of assembling a gallium arsenide blockonto a silicon substrate for illustrative purposes only. As shown, theinvention may be applied to forming gallium arsenide diodes onto siliconsubstrates. Another commercial application includes gallium arsenidelasers assembled with silicon integrated circuits. The silicon chips cancommunicate with other chips with integrated optical detectors onextremely high bit-rate optical channels. Other applications may alsoinclude integration of microwave gallium arsenide devices onto siliconintegrated circuits for the purpose of microwave electronics. Still afurther application includes microstructures integral with a plasticsheet forming active liquid crystal displays (ALCD) and the like. Insuch application, the plastic sheet may be fabricated by a techniqueincluding stamping, injection molding, among others. Advantages of theinvention include for example the applicability of conventionalmetallization or other processes over the planar surface forinterconnection of the integrated electronic devices or portions ofelectronic devices between the blocks and/or between blocks and otherelectronic devices on the substrate. The concept of the invention can beused with almost any type of microstructure which assembles onto alarger substrate.

To demonstrate the general operation and effectiveness of the method andapparatus for various applications such as those requiring differentsizes of shaped blocks, further examples of the invention involvedassembling microstructures on a silicon substrate using shapedtrapezoidal blocks of silicon having two different sizes and differingby 2.5 orders of magnitude in mass.

In the first silicon experiment, larger trapezoidal blocks were designedto have two-fold rotational symmetry with the size of the larger facehaving a dimension of about 1.0 mm×1.2 mm and a depth of about 235 μm,and recessed regions were designed to correspond to the dimensions ofthe larger trapezoidal blocks. In the second silicon experiment, smallertrapezoidal blocks were designed to have four-fold symmetry with thesize of the larger face having a dimension of about 150 μm×150 μm and adepth of about 35 μm, and recessed regions were designed to correspondto the dimensions of the smaller trapezoidal blocks. In bothexperiments, the method and apparatus exhibited high fill-factors.

In each of the silicon examples, the mixture or slurry including siliconshaped blocks and an inert fluid are transported over a top surface of asilicon substrate having complementary recessed regions by use of anapparatus that circulates the slurry over the substrate.

In creating silicon substrates having recessed regions for each siliconexperiment, a solution of potassium hydroxide (KOH) produced recessedregions having a trapezoidal profile or inverted truncated pyramidshape. The solution created trapezoidal shaped profiles or invertedtruncated pyramid shapes having a larger face with lateral dimensions atthe top surface with sides sloping inward at about 55° from the topsurface of the substrate. Trapezoidal profiles occurred due to theselectivity (1:200) between the {111} plane and the {100} plane.Specifically, the {111} plane etched slower than the {100} plane by theapproximate ratio of 1:200.

In the silicon examples, the KOH solution etched trapezoidal recessedregions into about 500 μm thick silicon wafers (5 cm×5 cm in overallarea) using a mask consisting of rectangular openings. The KOH etchantsolution used in the examples was a 1:2 (by weight) KOH:H₂ O solution atabout 80° C. Prior to the etching step, a silicon nitride (SiN_(x)) masklayer was first formed on a top surface of such substrates.

Masking and etching such silicon nitride layer formed trapezoidal shapedrecessed regions. The depth of the trapezoidal recessed region isdetermined by the length of time the silicon substrate is etched. If thesilicon is etched the correct length of time, recessed regions can bemade which are identical in shape and size to the blocks.

For the first silicon experiment, the larger recessed regions in thesubstrate had lateral dimensions of about 1.0 mm×1.2 mm at the topsurface and a depth of about 235 μm after about 23 minutes of etching tocomplement the larger trapezoidal shaped blocks. In total, the mask inthis experiment consisted of 191 holes corresponding to recessedregions, arranged in various test patterns.

For the second silicon experiment, the smaller recessed regions inanother substrate had lateral dimensions of about 150 μm×150 μm at thetop surface and a depth of about 35 μm after about 30 minutes of etchingto complement the smaller trapezoidal shaped blocks. The mask in thisexperiment was simply an array of 64×64 holes (corresponding to 4,096recessed regions) with about a 300 μm spacing between consecutive holes.

In the substrates for both silicon experiments, the silicon nitride masklayer was not removed from the substrates and remained on the substratesthroughout the microstructure assembling method.

In fabricating the larger trapezoidal shaped blocks for the firstsilicon experiment, a single-side-polished two inch silicon wafer ofabout 235 μm thickness provided a substrate for the formation ofself-aligning shaped blocks. In this experiment, the silicon waferitself is the block layer from which the trapezoidal shaped blocks areformed. A SiN_(x) layer of thickness about 0.4 μm was deposited on boththe polished bottom surface of the wafer and the unpolished uppersurface of the wafer. The SiN_(x) layer overlying the unpolished uppersurface of the wafer forms a sacrificial layer, and the SiN_(x) layeroverlying the polished bottom surface of the wafer forms a mask layer.The SiN_(x) mask layer overlying the polished bottom surface of thewafer was then patterned with photoresist.

Patterning the SiN_(x) mask layer overlying the polished bottom surfaceof the wafer included spreading a photoresist layer over this SiN_(x)layer. Patterning steps also included at least exposing, developing, andbaking the photoresist. Such baking step occurred at a temperature ofabout 120° C. for about 20 minutes to hard-bake the photoresist layer.The patterning steps formed a plurality of squares (or rectangles).After patterning, exposed regions were etched forming trapezoidal shapedblocks attached to the SiN_(x) sacrificial layer.

In fabricating smaller trapezoidal shaped blocks for the second siliconexperiment, a SOI wafer provided a substrate for the formation ofself-aligning shaped blocks. The particular SOI wafer used had a SOIlayer of about 35 μm thickness with a silicon dioxide (SiO₂) layer ofabout 0.4 μm thickness separating it from the rest of the wafer. The 35μm SOI layer is the block layer of silicon from which the trapezoidalshaped blocks are formed. The SiO₂ layer of about 0.4 μm thickness isthe sacrificial layer. A SiN_(x) mask layer of thickness about 0.4 μmwas deposited on both 35 μm silicon block layer and patterned withphotoresist.

Patterning the SiN_(x) mask layer overlying the upper surface of thewafer included spreading a photoresist layer over this SiN_(x) masklayer. Patterning steps also included at least exposing, developing, andbaking the photoresist. The patterning steps formed a plurality ofrectangles each having a dimension of about 150 μm×150 μm (exposedportions of the photoresist). After patterning, unexposed regions wereetched forming trapezoidal shaped blocks attached to the SiO₂sacrificial layer.

The slowest etching planes for silicon in this KOH:H₂ O etching solutionare the {111} planes, which can be considered etch stops forming thesloping sides of the shaped blocks. In each silicon experiment, therespective mask used to define the blocks must be aligned to theappropriate crystal axis. As shown in FIG. 100, a mask was used to formsilicon trapezoidal shaped blocks. Trapezoidal shaped block is formed atthe intersection of the diagonal lines on the mask. The width of thediagonal lines in the figure must be twice the thickness of the siliconblock layer. For the larger trapezoidal blocks, a=0.2 mm and t=235 μm;whereas, for the smaller trapezoidal blocks, a=0 and t=35 μm.

Etching is completed when this silicon block layer is etched entirelythrough, and simultaneously when the corners are precisely formed.Continuing etching beyond this point does not change the overalldimensions of the trapezoidal shaped block, but merely rounds thecorners. Because of geometric considerations, the width of the top faceof the shaped block must be at least 3√2 times the thickness of thesilicon block layer. This limits the aspect ratio of the blocksfabricated by this technique. This mask pattern utilizes as high as 50%of the silicon area if there is no distance between block corners.

In the first silicon experiment with the larger shaped blocks, theetched silicon wafer is placed in concentrated HF etch solution toremove the shaped blocks from contact with the SiN_(x) sacrificial layerand any remaining SiN_(x) from the mask layer. In the second siliconexperiment with the smaller shaped blocks, the etched SOI wafersimilarly is placed in concentrated HF etch solution to remove theshaped blocks from contact with the SiO₂ sacrificial layer and anyremaining SiN_(x) from the mask layer. This HF etch solutionpreferentially etches the SiO₂ and the SiN_(x) to free the shaped blockswithout etching the silicon shaped blocks. In particular, a HF solutionhaving a concentration of about 1:1 HF:H₂ O was used to etch thesacrificial layer and residual SiN_(x) to free the shaped blocks.

Once all the shaped blocks were free in the solution, the HF wasdecanted and water was added. For the first silicon experiment, waterand the larger shaped blocks formed the mixture or slurry. However, forthe second silicon experiment, methanol and the smaller shaped blocksformed the mixture or slurry. As silicon is hydrophobic in nature,smaller shaped blocks having smaller mass tended to float on the surfaceof the water. The larger blocks did not have this problem because oftheir larger mass. Since methanol has both polar and nonpolarproperties, the smaller shaped blocks did not float but preferablysubmerged into the fluid. As the silicon examples demonstrate, theselection of the fluid used in the slurry may depend on the mass of theshaped block, among other factors.

In the first silicon experiment, the mixture including approximately 500of the larger shaped blocks and water (about 0.5 liter) was placed intothe apparatus (about 4" diameter). The substrate was about 5 cm×5 cm andcontained 191 recessed regions in various patterns. Initially, smallbubbles tended to stick to the larger shaped blocks and to the recessedregions on the substrate, due to the fact that all the silicon surfacesare hydrophobic. Adding a small amount of surfactant of about 5 drops tothe water (1:100,000) made the silicon surfaces less hydrophobic andbubbles no longer stuck to the shaped blocks or to the recessed regions.The substrate was oriented at an angle which caused the incorrectlyoriented blocks to slide off. Agitation and correct orientation of thesubstrate increased the rate at which the blocks filled the holes.Because of the tapered shape of the trapezoidal blocks, nearly all theblocks descended through the fluid with the larger surface facingupward. Upon landing on the substrate, the blocks were thus properlyoriented to fill the recessed regions, resulting in enhanced fill rates.On repeated runs of this first silicon experiment, all 191 recessedregions were filled for a fill-factor of 100% in about 4.5 minutes.

In the second silicon experiment, the mixture including approximately30,000 of the smaller shaped blocks and methanol (about 0.5 liter) wasplaced into the apparatus. The substrate was about 5 cm×5 cm andcontained a 3 cm×3 cm array of 64×64 (4,096) recessed regions in thecenter of the substrate. Operating the apparatus in a manner similar tothat used for the first silicon experiment with the larger shaped blocksresulted in a saturated fill-factor of about 70% after about 15 minutes.

When the larger faces of the shaped blocks are rough, higherfill-factors were achieved. For shaped blocks having smooth largerfaces, the shaped blocks tended to stick to the smooth surface of thesubstrate and reduce the overall motion of the blocks on the substrate,with the fluid flow being insufficient to free blocks stuck in thismanner. In the first silicon experiment, the larger shaped blocks didnot have this problem since the larger face of the blocks was the roughback of the unpolished wafer. Shaped blocks having features, such ascircuits or contact pads, on the larger face have such rough surfacesfor higher fill-factors.

Roughening the top surface of the substrate having the recessed regionsalso leads to higher fill-factors. After removing the SiN_(x) masklayer, the substrate was lapped in about 0.5 μm Al₂ O₃ lapping powderjust long enough (about 1-2 minutes) to create about a 0.3 μm roughnesson the surface. Blocks in all orientations easily came off the substrateafter it was roughened. Using a lapped substrate in the second siliconexperiment with smaller shaped blocks, saturated fill-factors of about90% were achieved after about 15 minutes in the apparatus. Afterrepeated runs of the second silicon experiment, it was determined thatthe 90% fill-factor achieved for the smaller blocks was a steady statecondition.

In order to achieve even higher fill-factors, especially for the smallershaped blocks, care must be taken to properly etch the substrate and tokeep the substrate and solution free of particles that could interferewith filling. For example, interference with filling could occur if thebottoms of the recessed regions are left rough by the KOH etching, or ifparticulates are deposited inside the recessed regions during thelapping process. Nonuniformity in the bottoms of the recessed regionscould either prevent blocks from entering the recessed regions, or aidin the unfilling of the blocks from these recessed regions.Additionally, substrates with recessed regions that were not etched deepenough did not achieve high fill-factors, since the blocks protrudingabove the edge of the recessed region in the substrate could more easilybe removed by fluid flow. Substrates with recessed regions that wereetched too deeply also did not achieve high fill-factors. While thelateral dimensions at the top of an overetched recessed region remainthe same, the lateral demensions at the bottom of the recessed regionbecome smaller as the depth is increased. Therefore, only the edges andnot the smaller bottom face of a block filling an overetched recessedregion will touch the substrate. Optimally, higher fill-factors areachieved when the recessed regions are neither underetched noroveretched.

Also described in general terms is the unique profiles for creatingself-assembling devices. Such unique profiles, for example, are terms ofa single block structure having a corresponding recessed regionstructure on a substrate for illustrative purposes only. The blockstructure may also include a variety of shapes such as a cylindricalshape, rectangular shape, square shape, hexagonal shape, pyramid shape,T-shape, kidney shape, and others. The block structure includes widths,lengths, and heights to promote self-assembly for a desired orientation.In addition, more than one type of structure may be present in themixture (solution and blocks) as long as each structure includes aspecific binding site on the substrate.

Although the foregoing invention has been described in some detail byway of illustration and example, for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

The above description is illustrative and not restrictive. Manyvariations of the invention will become apparent to those of skill inthe art upon review of this disclosure. Merely by way of example theinvention may used to assemble gallium arsenide devices onto a siliconsubstrate as well as other applications. The scope of the inventionshould, therefore, be determined not with reference to the abovedescription, but instead should be determined with reference to theappended claims along with their full scope of equivalents.

What is claimed is:
 1. Apparatus for assembling a microstructure on asubstrate with at least one recessed region thereon, said apparatuscomprising:a vessel comprising a substrate therein, said substratecomprising a recessed region thereon; and a circulation system coupledto said vessel, said circulation system circulating a fluid comprising aliquid which contains a microstructure in the form of a plurality ofshaped blocks at a rate where at least one of said shaped blocks isdisposed into said recessed region.
 2. Apparatus of claim 1 wherein saidvessel comprises an agitation system to agitate said substrate. 3.Apparatus of claim 1 wherein said vessel comprises a means formechanically agitating the fluid with ultrasonic vibration.
 4. Apparatusof claim 1 wherein said vessel further comprises a receptacle with afunneled bottom; and a conduit having an input, said input coupled tosaid funneled bottom and to said circulation system, a column coupled tosaid input, and an output coupled to said column and leading to saidreceptacle.
 5. Apparatus of claim 4 wherein said receptacle furthercomprises a holder for moving said substrate to facilitate dispositionof said shaped block into a recessed region.
 6. Apparatus of claim 5wherein said holder is coupled to said substrate.
 7. Apparatus of claim5 wherein said holder agitates said substrate so that said shaped blocksfree from said recessed regions can flow off said substrate. 8.Apparatus of claim 1 wherein said circulation system comprises a pumpcoupled to said conduit, said pump dispensing a gas to facilitate thecirculation of said shaped blocks in said vessel.
 9. Apparatus of claim8 wherein said pump is adjustable to provide a selected rate tocirculate said shaped blocks.
 10. Apparatus of claim 8 wherein said gasincludes nitrogen.
 11. Apparatus of claim 8 wherein said pump isadjustable to achieve different fill-factors of the recessed regions.12. Apparatus of claim 8 wherein said gas forms bubbles within saidfluid.
 13. Apparatus of claim 1 wherein said circulation system canrecirculate said shaped blocks.
 14. Apparatus of claim 1 wherein saidshaped blocks are self-aligned into said recessed regions.
 15. Apparatusof claim 1 wherein said shaped blocks are bonded to said substrate. 16.Apparatus of claim 1 wherein said shaped blocks flow over said substrateuntil a fill-factor is achieved.
 17. Apparatus of claim 1 wherein saidvessel is made of a glass material.